Although the rents in the city centre a lot higher, this is for a reason, the potential of footfall customers in the city centre is far higher than anywhere else in Dublin, all rail and bus links travel to the city centre, if you got somewhere like the lads were saying, in smithfield you would have potential for on street parking also,
I would stay clear of Blanch and Sandyford as these are the extremes of west and south dublin and you dont want to loose or alienate customers due to people from the northside etc who wont be able to reach your store without a car.